1. Field of the Invention
This invention relates to a photosensitive coating composition which can be cured upon exposure to actinic radiation such as ultraviolet light. More particularly, it relates to a photosensitive coating composition comprising (1) a prepolymer containing radically cross-linkable ethylenically unsaturated double bonds and (2) a sensitizer which comprises predominantly a mixture of (a) benzophenone, a halogenated benzophenone or a mixture thereof and (b) 4,4'-bis(diethylamino)benzophenone in a weight of about 1:2 to 10:1.
The "photosensitive coating composition" as used herein is, for example, a ultraviolet light curable type coating varnish or a printing ink containing various pigments, waxes, monomers, stabilizers and the like as optional ingredients which are commonly used in varnishes and printing inks of this type.
2. Brief Description of the Prior Art
Recently there has been the tendency toward eliminating solvents from coating compositions in order to solve possible pollution problems such as air pollution, and extensive investigations are now underway for non-solvent type coating compositions which solidify (dry or cure) on a coated surface. The results of some of these investigations are disclosed in U.S. Pat. Nos. 3,551,235, 3,551,246, 3,551,311, 3,552,986 and 3,558,387, British Pat. No. 1,198,259, Japanese Patent Publication Nos. 10606/57 and 35688/71 and Japanese Patent Laid Open Nos. 38202/72 and 20287/72 and so on. Some of these non-solvent type coating compositions have been tested on their marketability in the printing field, but at present such coating compositions do not have a great market share over the conventional printing inks due to their disadvantages, for example, poor storage stability when the sensitivity is increased and the like.
Coating compositions capable of being cured by irradiation with ultraviolet light, i.e., printing inks or coating varnishes, require a prepolymer containing radically crosslinkable ethylenically unsaturated double bonds and a sensitizer as essential ingredients.
Hitherto many sensitizers are known to generate radicals upon exposure to light. Of these sensitizers, ketone-type sensitizers such as benzoin methyl ether, benzoin ethyl ether, benzoin butyl ether, .alpha.-methylbenzoin, .alpha.-allylbenzoin, .alpha.-chlorodeoxybenzoin and the like, azo compound sensitizers such as azobisisobutyronitrile and the like, quinone-type sensitizers such as anthraquinone, phenanthraquinone and the like, sulfide compound sensitizers such as tetramethylthiuram disulfide and the like are widely employed for plating plates, photoresists or photosensitive paints.
However, particularly in the printing field, it is required that a relatively thick liquid coating film of, for example, ultraviolet light-curable type printing ink or coating varnish be solidified (dried or cured) upon irradiation with ultraviolet light in quite a short period of time, and that, on the contrary, the coating composition must be stable without undergoing a dark reaction on a printing or in a coating machine or in a container. The above described sensitizers increase relatively, but not sufficiently, the sensitivity of the prepolymer when their proportion to a prepolymer is increased. However, the dark reaction is also promoted in proportion to a degree of the sensitization thereby frequently resulting in objectionable phenomena such as gelation of the composition during storage in a container or during operation on machines. Therefore, it is very important to develop a sensitizer which has the ability for high sensitization but no tendency toward causing the dark reaction.